Transmissible Spongiform Encephalopathies (TSEs) are a group of progressive neurodegenerative diseases that emerge as three basic types: the inherited familial forms, the traditional, long-standing sporadic forms, and the new variant forms.

The inherited familial forms are strongly linked to a mutation on the prion protein gene, although environmental factors are involved in triggering the TSE. The profile of these TSEs is characterized by a slow degeneration of the central nervous system, ultimately ending in dementia, motor difficulties and death. Gerstmann-Straussler-Scheinker disease progresses slowly over about five years; Fatal Familial Insomnia begins with bizarre sleep and sexual disturbances and rapidly progresses to a fatal chronic insomnia lasting just a few months.

The traditional forms of TSEs appear in older mammals and have a variety of names depending on the species of animal: scrapie in sheep and goats, chronic wasting disease in deer and elk, transmissible mink encephalopathy in mink, and sporadic Creutzfeldt Jakob disease (CJD) in humans. These sporadic forms of TSE exhibit a brain pathology that is characterised by spongiform degeneration and loss of neurons in many regions of the brain (such as the cortex), gliosis, shrunken basal ganglia and prion rods (fibrils of aggregated proteins, mainly composed of misfolded prion protein). Symptoms involve behavioural and cognitive disorders progressing to visual, motor and sensory disorders, then ataxia, muscle wasting, seizures, insanity and death.

The modern new variant forms appear as Bovine Spongiform Encephalopathy (BSE) in young cows, cats, antelopes and new variant Creutzfeldt Jakob disease (vCJD) in young humans. These new variant forms involve an accelerated, more aggressive course of the disease involving a more widespread unique neuropathology characterized by florid plaques (plaques surrounded by spongiform holes) with more pronounced psychiatric
disorders.